Bad chain line and cross chaining are big causes of it. Non-existant for properly maintained, set up, and shifted set ups. Clutched RDs helps a bit too.

Bad chain rings is a cause for those with miles on them. Teeth end up hooked so they dont release the chain.

Not sure what year ex5 you have, so not sure if it has 2x or 3x crankset, what cassette set up you have etc. Im going to guess 2x drivetrain since you posted in the 27.5 forum (though should be asking in drive train forum)

Ya I have the 2 by 8 i believe and didn't notice a drivetrain forum. The first chain suck incident was less than 1 mile on the odometer. Granted I changed gears under load but still. I've never had any other bike do this like this fuel ex5

In my experience, chain suck can be caused by a number of reasons, but the one I most commonly see is dirt and gunk accumulation. A muddy or wet ride, messy lube and/or applying it wrong will cause chain suck sooner or later.

I had constant chain suck in dirty or wet rides on one of my bikes with a 22 tooth granny ring. The chain suck only happened on the small chainring only, and always by the end of the ride after several stream crossings. I was about to change the chain ring to a 24 but I tried changing to squirt lube first. I never had another issue with chain suck after switching to squirt.

i have had chain suck many times... and every time the problem has been a worn front chain ring... as said before, the teeth become hook shaped and grab onto the chain instead of freely releasing it...